Tip: How to determine your wheel’s width and offset

We bought a set of wheels, but didn’t know their width and offset. All we know is that the wheels are 15″ in diameter and bolt pattern is 4×114.3.

While they will bolt on to any one of our Project Elantras, we needed to know if they were dimensionally similar to the other wheel sets we will be running. Offset affects an axle’s track width, which can play a part in ultimate roadholding and suspension tuning.

Usually size and offset information of a wheel is engraved or embossed at the back side of its spokes. Sometimes it is indicated on a sticker or label. But none of those were present.

So we brought out two of our most sophisticated measurement devices in order to know the wheel’s specs.

A ruler and a straightedge is all you need to determine wheel width and offset. We used the silver ruler because the lines were more legible on it than the clear plastic one. The piece of 2×2 coco lumber was not that straight but it would do.

First we needed to measure the wheel’s width. Insert the ruler as shown against the inner barrel of the wheel. Align the “0” line against the edge of the front (or rear) lip.

See how many inches the opposing lip is from the other. Deduct 1″ from your measurement. The actual wheel width is where the bead of the tire contacts the rim. The wheel lips are usually half an each thick each.

The lip to lip width is exactly 7″. This means the wheel is 6″ wide.

Now you got the wheel width. It is almost always measured in inches. Now, we need to figure out wheel offset. We do this by measuring the wheel backspacing.

Backspacing is the location of the wheel mounting surface from the outer rim. Offset is the location of the surface from the centerline of the wheel. On most wheels the offset will be a number with a prefix ET, which is short for the German Einpresstiefe, or insertion depth.

Backspacing  will almost always be a positive number. Offset can be positive (wheel more inboard), negative (wheel more outboard) or zero. Backspacing is usually measured in inches. Offset is given in mm.

Backspacing vs. Offset. Image credit: gtsparkplugs.com

Place your straightedge against the inner lips of the wheel, bisecting it. Place your ruler’s “0” mark against the edge of the mounting surface. Measure how many inches your backspacing is.

The straightedge must be as flat against the wheel edges as possible. Lucky the tire sidewall did not balloon outward too much as to hamper measurement.
Then you align the ruler in this manner so you can measure the backspacing. The wheel has 5″ backspacing

Then its just a simple math equation to convert backspacing to offset.

Offset = (wheel width – backspacing + 0.5) * 25.4.

It turns out that the wheels are a 15×6 with offset 38. they compares well with our 15×6.5 ET40 set of Rota Slipstreams, with minimal track loss. If the wheels were wider, say a 6.5″ or even 7″, we would be happier. But they will do.

Next time we might discuss how to determine wheel bolt pattern and center bore. We’ll keep you posted.

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